Scotland has set itself the world's most ambitious greenhouse gas reduction targets after the Scottish parliament voted today to cut the nation's CO2 emissions by 42% by 2020.

In a rare show of unity, all political parties at Holyrood unanimously agreed to fix the target as part of a radical climate change bill which also requires the Scottish government to set legally binding annual cuts in emissions from 2012.

The campaign coalition Stop Climate Chaos Scotland, which claims its 60 member organisations represent two million people, said this "hugely significant" vote set a new "moral" standard for the rest of the industrialised world.

Kim Carstensen, head of WWF International's global climate initiative, said: "At least one nation is prepared to aim for climate legislation that follows the science. Scotland made the first step to show others that it can be done. We now need others to follow."

However, the new measures are already under intense scrutiny. The act allows ministers to reduce the target later this year if the UK government's advisory panel on climate change says it is unrealistic, or the UN climate change conference in Copenhagen in December fails to agree on a global deal to replace Kyoto.

Environment groups are critical of the Scottish government's refusal to abandon road, bridge and airport expansion programmes, its plans for a new coal-fired power station, and its unwillingness to tackle directly increasing car use.

Furthermore, Scottish ministers only directly control about 30% of Scotland's total annual emissions of 68m tonnes of CO2 – which only equates to a 700th of the world's emissions. Most significant policies are controlled in Brussels and London, critics point out.

About 40% is covered by the European Union carbon emissions trading agreement, while the UK government has policy responsibilities for a further 30% of Scotland's emissions. That includes fuel taxation, low emission vehicles, VAT on energy efficiency and air taxes.

The Committee on Climate Change, the panel set up to advise Gordon Brown's government, has warned Salmond that Scotland is effectively jumping the gun by setting a 42% target in advance of a deal at Copenhagen.

In a letter to Stewart Stevenson, the Scottish climate change minister, the committee's chief executive, David Kennedy, said it believes Scotland should follow the UK strategy of waiting until the Copenhagen conference.

If a deal is reached, it should follow the UK government's lead and only then set a 42% target.

The Scottish government had also increased the pressure on itself by including emissions from international aviation and shipping in its target, Kennedy wrote, even though it has no control over policy for these sectors.

"I would therefore consider that an appropriate Scottish 2020 target could be set slightly below 34% to account for different treatments of international aviation under UK and Scottish approaches."

Despite these criticisms, the chairman of Stop Climate Chaos Scotland, Mike Robinson, said the significance of the all-party consensus could not be underestimated.

"It means Scotland's climate change bill has the toughest target of any industrialised nation in the world and will be held up as an example, ahead of the climate talks in Copenhagen in December, of what can and should be done," he said.

"This is a moral commitment and we hope other developed nations will hear this call for action and follow Scotland's lead."

Although on renewable energy the Scottish National party is very likely to surpass its ambitious targets to deliver half of Scotland's electricity from renewables by 2020, ministers have failed to embark on any politically unpopular measures to combat car use or the growth in short-haul aviation.

It has authorised a second road bridge over the Firth of Forth and abandoned bridge tolls, paid to extend the M74 motorway, supports a new ring road around Aberdeen and dualing the A9 and wants a major new coal-fired power station.

Its most ambitious emissions-reduction policies, such as using carbon capture for all fossil fuel power stations, using marine energy, and a wholesale switch to green transport, either have targets set at 2030 or are largely UK-government controlled. The SNP has also completely ruled out any new nuclear power stations.